With growing global interest in eco-friendly businesses, demand for clean fuel, which can replace existing energy sources such as petroleum and coal, is increasing. In this situation, natural gas is used in various fields as a main energy source having cleanliness, stability and convenience. Unlike in the US and Europe, where natural gas is directly supplied through pipelines, Korea introduced liquefied natural gas (LNG) obtained by liquefying natural gas at an extremely low temperature and has supplied LNG to consumers. Thus, the demand for a cargo containment system (CCS) for storing LNG is increasing along with the increase in domestic natural gas demand.
LNG is obtained by cooling natural gas to an extremely low temperature (about −163° C.) and is suitable for long-distance transportation by sea since LNG is significantly reduced in volume, as compared with natural gas in a gaseous state. LNG carriers are designed to carry liquefied gas to an onshore source of demand and, for this purpose, include a cargo containment system capable of withstanding ultra-low temperatures of LNG.
Such a cargo containment system is divided into an independent tank-type and a membrane-type depending on whether the weight of cargo is directly applied to an insulator. The membrane-type cargo containment system is divided into a GTT NO 96-type and a Mark III-type, and the independent tank-type cargo containment system is divided into an MOSS-type and an IHI-SPB-type. The GTT NO 96-type and GTT Mark III-type were formerly called a GT type and a TGZ type. After, in 1995, Gas Transport (GT) and Technigaz (TGZ) were renamed to GTT (Gaztransport & Technigaz), the GT type and the TGZ type have been referred to as the GTT NO 96-type and the GTT Mark III-type, respectively.
A membrane-type LNG cargo containment system consists of double bulkheads. Here, a primary sealing wall is mainly formed of metal. Typically, a primary sealing wall of a GTT NO 96-type cargo containment system is formed of Invar and a primary sealing wall of a GTT Mark III-type cargo containment system is formed of Steel Use Stainless (SUS). In addition, a secondary sealing wall of a GTT NO 96-type cargo containment system is formed of Invar and a secondary sealing wall of a GTT Mark III-type cargo containment system is formed of Triplex, which is a non-metal.
Invar and Triplex are materials that hardly undergo thermal deformation, whereas SUS is a material that is subject to relatively severe thermal deformation. Thus, unlike a sealing wall formed of Invar or Triplex, a sealing wall formed of SUS must have wrinkles to cope with heat shrinkage near −163° C., which is the temperature of LNG.
FIG. 1 is a schematic perspective view of a primary sealing wall of a GTT Mark III-type LNG cargo containment system.
Referring to FIG. 1, each side of the primary sealing wall 100 formed of SUS is welded to an upper surface of an anchor strip 500 secured to an upper surface of a primary heat-insulating layer 200. In the primary sealing wall 100, each of four sides is secured to the anchor strip 500 and there are no other securing points on the surface of the primary sealing wall. Thus, the primary sealing wall uniformly shrinks upon temperature decrease such that wrinkles formed on the primary sealing wall can function properly.
However, each side of a secondary sealing wall welded to an upper surface of an anchor strip is secured on an upper surface of a secondary heat-insulating layer, and the secondary sealing wall has other securing points connected to the primary sealing wall 100. Thus, the secondary sealing wall does not uniformly shrink upon temperature decrease such that wrinkles formed on the secondary sealing wall cannot function properly.
Therefore, although SUS has more competitive price than Invar and has superior advantages over Triplex in terms of air-tightness, a typical LNG cargo containment system has a problem in that the use of a secondary sealing wall formed of SUS is limited.